A typical electromagnetic radiation source using magnetic fields, such as a free electron laser, uses a series of permanent or electromagnet elements arranged axially with an axial cavity having a transverse magnetic field therein. The transverse magnetic field alternates in direction axially along the tube. Accordingly, an electron passing along the axial direction in the cavity is subject to a repetitive motion. This motion constitutes acceleration which will cause the electron to radiate electromagnetic radiation with a frequency depending on the period of the repetitive motion or the alternating magnetic field. One such magnetic structure that could be used to provide such an alternating magnetic field within a permanent magnet structure is disclosed in U.S. Pat. No. 4,862,128 entitled "Field Adjustable Transverse Flux Sources" issuing to Herbert A. Leupold on Aug. 29, 1989, which is herein incorporated by reference. When such a structure is formed to provide a magnetic field that is transverse to the axis and alternates with progression along the axis, an electron beam traveling along the longitudinal axis will oscillate in the direction normal to both the field and the longitudinal axis. A plane-polarized electromagnetic wave emanates from the accelerated electrons with the waves electric vector in the direction of the acceleration. By proper adjustment of the field strength and structural period, the various parts of the configuration can be made to radiate coherently. When this happens, the structure, which is generally referred to as a wiggler, is called an undulator and laser action occurs. However, in a linear structure, energy is extracted from an electron beam in the form of the desired electromagnetic radiation only during the time the electron beam transverses the spatially alternating magnetic field. The average magnetic field in such a magnetic structure is zero, because of the reversal between opposite polarities of equal strength.
While various magnetic structures have been designed to produce the desired forms of electromagnetic radiation, due to the limited time in which the electron beam transverses the spatially alternating magnetic field in a linear permanent magnet tube only a small portion of the kinetic energy of the electron is converted to electromagnetic radiation. Accordingly, there is a need for a permanent magnet structure that can adequately contain the electron beam in an effort to increase the power of the electromagnetic radiation thereby created.